Vapor pressures, molar enthalpies of sublimation, and molar enthalpies of solution in water of selected amino derivatives of uracil and 5-nitrouracil

Vapor pressures and enthalpies of sublimation of crystalline 5-aminouracil, 6-aminouracil, 6-amino-1-methyluracil, 6-amino-1,3-dimethyluracil, and 5-nitrouracil were measured by the Knudsen effusion method. The heats of solution in water were determined at 298.15 K over the range of concentrations (0.828 to 1.845) mmol.kg(-1) by using an isoperibol solution calorimeter. The molar enthalpies of solution at infinite dilution were evaluated. The molar enthalpy of hydration was calculated by subtracting the estimated molar enthalpy of sublimation and the molar enthalpy of solution. The contributions of the -NH2, -NO3, and -CH3 groups to the molar enthalpies of solution, sublimation, and hydration were calculated by applying a simple additivity scheme. The molar enthalpies of hydration were correlated with the structural parameters of the compounds investigated. They were found to be linearly related to the polar and to the apolar parts of the accessible surface areas of the compound studied.